Photosynthesis, Nitrogen Levels, and Dry Matter Accumulation in Flag Wheat Leaves During Grain Filling

Abstract

Some relationships between net photosynthesis rate, gas exchange resistan ces, nitrogen concentration and dry matter content have been studied in flag leaves of spring wheat (Triticum aestivum L, cv Kolibri) during grain filling. Plants were grown in the field, under Mediterranean climate conditions, and the gas exchange measurements were also made in the field. Net CO₂ assimilation rate at ambient CO₂ and O₂ levels and leaf nitrogen concentration presented the maximum values at about anthesis (day 117 after seeding) and then declined slowly until the onset of senescence (about day 20 after anthesis). In senescent leaves, these parameters declined much more rapidly with time. Conversely, the specific dry weight (SDW) significantly increased until the start of senescence, and then declined also very rapidly. Thus the highest SDW values (62 g DW/m²) occurred at about the same day in which photosynthesis and nitrogen content started to decrease very rapidly. Specific dry weight changes in mature wheat leaves were mostly accounted for by changes in the assimilate levels, and the progressive increase of SDW values before the senescence onset presumably reflects an unbalance between assimilate production and utilization. Changes in the stomatal resistance were also monitored and they were not primarily involved in the photosynthesis decline during senescence. The increase in the residual resistance to CO₂ (which mainly includes the carboxylation resistance) seems to be the main factor affecting photosynthesis rate during grain filling. The magnitude of this resistance was several times larger than that of the stomatal resistan ce, and this magnitude greatly increased during senescence. Possible relationships between assimilate (e.g. carbohydrates) accumulation and leaf senescence will be discussed.